Representative, for instance, of some kinds of development activities occurring in the past 10 years for reducing the weight of automobiles and thereby enabling the autoist to be more conservative of fuel, would appear to be the following.
According to the May 1969 issue of "Auto-products", the G.R.T.L. Company, a joint development venture of PPG Industries, Inc. and Union Carbide Corporation, was formed (about 1968) to produce a family of glass-reinforced thermoplastic sheets called "Azdel" which could be formed on conventional metal-stamping apparatus for the purpose of meeting the automobile industry's need for high-speed productivity, for example 180 to 360 parts per hour with a single press. The Azdel glass-reinforced thermoplastic sheet can be formed in one operation into shapes that take four or more separate stamping operations when working with a sheet of steel. The Azdel sheet made from polypropylene is preheated in an infrared oven to about 400.degree. F. (about 204.degree. C.) then fed into a press and formed between cooled matched metal dies. The operation, according to the article, is scrap-free and as the stamped Azdel sheet comes from the mold it has no flash or trim, and holes and notches can be formed in the stamping operation. Shapes can be stamped from Azdel sheets that would be impossible in steel; and in some cases an assembly of several parts in steel can be redesigned so that it can be made in one part from an Azdel sheet.
Also according to the Jan. 22, 1968 issue of "Chemical and Engineering News", the Azdel glass-reinforced thermoplastic sheets reportedly could be made from styrene acrylonitrile copolymer (42% glass reinforced), polyvinyl chloride (36% glass reinforced), polypropylene (44% glass reinforced) or other resins. The Azdel sheet contains generally about 40% glass fiber by weight. Reinforced polypropylene has a heat distortion temperature of 327.degree. F. (about 164.degree. C.); styrene acrylonitrile copolymer, 255.degree. F. (about 124.degree. C.); and polyvinylchloride, 221.degree. F. (about 105.degree. C.). A given part can be formed from a variety of blank sizes, such as an 84 mil. thick hood can be formed from a 150 mil. thick by 100 sq. in. blank or from a 125 mil. thick by 121 sq. in. blank ("SPE Journal", September 1972, Vol. 28, pages 38-42).
Further, in the September 1976 issue of "Plastics World", page 53, there was a later announcement that a new grade of Azdel sheet based on PBT (polybutylene terephthalate) thermoplastic polyester reinforced with 30 weight percent of continuous glass fiber mat was being offered. These Azdel sheets were formed in a stamping operation after being preheated to 450.degree. F. to 500.degree. F. (about 232.degree. C. to about 260.degree. C.).
Allied Chemical Corporation, for instance, produces a stampable nylon 6 composite sheet which is registered under the name STX, and has a combination of about 50% nylon 6 resin, about 30% glass fiber reinforcements and about 20% fillers. The composite sheet must be heated to a temperature above its melting point before it can be stamp formed, according to an article in the March 1979 issue of "Plastics Engineering" (pages 47-49).
In every instance mentioned above, the preheating of the composite sheet apparently has to take place at or above the melting point of the thermoplastic material being used in the sheet. Such heating, of course, requires that a significant amount of energy be used for each sheet. Also heating at or above the melting point means that greater care must be exercised in transporting a sheet in its melt or above-melt state as from the infrared oven to the forming or stamping press.
One thermoplastic material that does not appear to be given as much mention in the literature for structural purposes as other thermoplastic materials is the polyester, poly(ethylene terephthalate). It is noted, for instance, in U.S. Pat. No. 3,547,891 that there is disclosed a thin film material or sheet material (about 7.5 to 10 mil. in thickness) of poly(ethylene terephthalate), that has been vacuum heat formed, starting and ending essentially in the amorphous state. This amorphous final state would apparently be suitable for the final product, as for use in blister packages, as mentioned in the patent, but not for use in the final form of automobile parts. Another patent, U.S. Pat. No. 3,496,143, discloses a process for vacuum deep-drawing of poly(ethylene terephthalate) sheet material, which must have a solution viscosity [as determined in a 1% solution of the poly(ethylene terephthalate) in meta-cresol at 25.degree. C.] of about 1.7 to about 2.0 and a degree of crystallization of at least 5% up to about 25%. Neither this sheet material nor the one disclosed in U.S. Pat. No. 3,547,891 is a reinforced material or one of laminate construction. U.S. Pat. No. 3,496,143 specifies that its vacuum-formed product is not amorphous and that it has a higher degree of crystallinity that the initial material being molded; the molded material also being considered as having a degree of crystallinity in the range of 5% to 25%.
U.S. Pat. No. 3,765,998 discloses a high-impact resin sheet which is formable in shaping apparatus held at ambient temperature and concerns a glass mat having a glass fiber length of at least one inch, impregnated with poly(ethylene terephthalate) having a weight average molecular weight from about 5,000 to about 45,000. The sheets are preheated from about 240.degree. C. to about 280.degree. C. and are then transferred to a mold or press where they are cooled slowly under pressure to develop crystallinity (Examples 1 through 8). Examples 9 and 10 speak of chilling the laminate sheet, but since there is no indication of the rate of chilling taking place, the state of crystallinity cannot be determined. In any event the patent teaches preheating to around the melt temperature for all examples. There is an indication in the specification that the "PET" [poly(ethylene terephthalate)] polymer has a level of crystallinity of from about 20% to about 60% as determined by X-ray techniques (column 3, lines 51-57), but it is not clear whether or not this statement has reference to the polymer in the pellet form prior to impregnation into the sheet or to the polymer when in the sheet form.
An advantage of the use of thermoplastic resin instead of the thermoset resin is that the former needs only to cool below its crystallization temperature before a stamping press can be reopened and the part removed. A thermoset resin part must be given time for a chemical reaction to occur in order to cure the part before it can be removed from a stamping press.
Another advantage is that a thermoplastic part can be recycled, if need be, by reheating, whereas a thermoset part cannot be recycled by reheating.
U.S. Pat. No. 4,263,364 discloses reinforced thermoplastic polyester sheets which can be rapidly quenched from the melt to a stable amorphous state. The quenched, amorphous sheets may then be stamp-formed at temperatures that are below the melting point (T.sub.m) of the polyester but above its glass transition (T.sub.g) temperature, or the quenched amorphous sheets may be stored and then stamp-formed at another time. In this manner, therefore, considerable energy will be saved as compared to some of the other prior art processes mentioned above involving the necessity of heating to or above the melt temperature of the polymer involved. Also the sheet may be more easily handled in moving it to a forming or stamping apparatus than one heated at or above the melt temperature. Further, flat amorphous sheets can be stored indefinitely at ambient temperatures until needed and then transported to a forming or stamping press. The resulting formed or stamped part retains the shape of the mold in the press and possesses an overall high set of properties such as surface appearance, heat distortion temperature, flexural and impact strength which qualify it for use in both appearance and structural applications. The resulting sheet will thus find utility in such applications as exterior automotive parts, which are exposed to elevated paint oven temperatures.
The reinforced thermoplastic polyester sheet thus may be produced and then reduced to an amorphous state and subsequently stamped and crystallized simultaneously at a temperature below the melting point of the polyester but above the glass transition temperature. The reinforced stampable thermoplastic polyester sheet of laminate construction has a center layer or layers which is or are comprised of a more slowly crystallizing polymer than the outer layers of the sheet. This construction reduces the need to rapidly quench the core of the sheet so as to form the amorphous sheet and thus enables higher production rates and the production of thicker amorphous sheets.
Some essential requirements of a stampable sheet that is to be used for exterior automotive-type appearance parts are that the sheet have a smooth surface and that it is essentially free of fiber "read-through". The latter occurs when the polymer layer above the layer of fibrous reinforcement, such as glass fiber reinforcement, crystallizes so fast that it shrinks around the glass fibers and pulls on them, resulting in a "read-through" of the fibers. Some polymers, however, which are effective in reducing or eliminating fiber "read-through" when used as layers in the sheet, also can cause a blistering of the surface to appear in the formed part. Such blisters represent another type of surface defect which must be avoided. Thus, sheet compositions are needed in which fiber "read-through" is eliminated and in which no blistering occurs during operations connected with forming a part from the stampable sheet.
An object of the invention, therefore, is to produce a stampable sheet which is subsequently stamped into parts which are highly crystalline and suitable for painting and baking and may be used for exterior automotive panels, the sheet being characterized by a smooth, glossy surface, an absence of fiber read-through, and high mechanical properties including a high heat deflection temperature.
Another object is to provide a stamped sheet from the stampable sheet, the stamped sheet having a heat deflection temperature under 264 psi. load greater than T.sub.m -50.degree. C. where T.sub.m is the melting point of the outer layers of the sheet.
On forming ribs or bosses or similar shapes from a stampable sheet where the primary reinforcement is due to the use of continuous fibrous reinforcing mats, resin-rich areas are generally produced in the outer regions of such ribs or bosses or similar shapes. Because of the lack of reinforcement in these regions, they will generally have lower mechanical properties than found in the other regions of the sheet.
Still another and more significant object of the invention, therefore, is to provide a stampable sheet that will have more uniform reinforcement in the sheet when it is subsequently formed into a stamped sheet in order to provide the necessary reinforcement in such ribs or bosses or similar shapes.
As mentioned above, the use of some polymers in the layers of a reinforced thermoplastic polyester sheet will, upon being subjected to heat in an infrared oven for subsequent stamping or forming of the sheet, result in blistering of that layer or layers in the formed part. This blistering also undesirably extends up through the sheet and is revealed on the surface of the sheet.
Still another object, therefore, is to provide a stamped sheet from the stampable sheet that is free from blistering. Other objects inherent in the nature of the invention will become apparent to those skilled in the art to which this invention pertains.